In a fiber-optic telecommunications network, a cable comprising several optical fibers can be used to produce several different transmission channels. It is also possible to carry out temporal data multiplexing in order to achieve the same purpose. However, where a further increase in datastream capacity of the network, the current trend is to transmit simultaneously, on the same optical fiber, several light wavelengths modulated independently of each other and each defining a data channel. ITU (International Telecommunications Union) Standard 692 proposes to define adjacent channels of 100 GHz optical spectral bandwidth that are centered on N adjacent normalized optical frequencies, the values of which are 200 terahertz, 199.9 terahertz, 199.8 terahertz, etc., corresponding to N wavelengths of 1.52 microns up to 1.61 microns. In a channel of this bandwidth, light can be modulated at 10 to 40 gigabits per second without too great a risk of interference with the channels of immediately adjacent spectral bands, (using modulation pulses of Gaussian waveform in order to minimize the bandwidth occupied by this modulation). This frequency multiplexing technique is called DWDM (dense wavelength division multiplexing).
In a telecommunications network, the problem is therefore to be able to collect the light corresponding to a given channel without disturbing light in the neighboring channels. For example, at a transmission node of the network, assigned to the transmission and reception of data of channel i, it must be possible for the light to be collected at a central frequency Fi (wavelength λi) without impeding the transmission of the light modulating the central frequencies F1 to FN, although these optical frequencies are very close to one another.
To do this, highly wavelength-selective optical filtering components must be produced that are capable of letting through the central optical frequency Fi and the frequencies lying within a narrow band of less than 50 GHz on either side of this frequency, and of stopping the other bands. At the output of such a filter, only light from channel i is collected and this light can be demodulated in order to collect the useful data.
It has already been proposed to produce filtering components that operate on the principle of Fabry-Pérot interferometers, produced by depositing semiconductor layers that are separated from one another by airgaps of thicknesses calibrated with respect to the wavelength λi to be selected. In practice, an interferometer comprises two mirrors consisting of superposed dielectric layers (Bragg mirrors) of high reflection coefficient that are separated by a transparent gap of optical thickness kλi/2 (if the gap is an airgap, this is the actual thickness kλi/2) where k is an integer defining the order of the interferometric filter. Indium phosphide (InP) is well suited to such constructions, especially because of its transparency at the wavelengths in question, its very high refractive index and the possibility of depositing epilayers of well-controlled thicknesses.
If the thicknesses of the layers and the gaps between the layers are very well controlled and if the materials have a high refractive index, such a filter proves to be highly selective.
Such a construction is described in the article by A. Spisser et al. “Highly Selective 1.55 micrometer InP/airgap micromachined Fabry-Perot filter for optical communications” in Electronics Letters, No. 34(5), pages 453-454, 1998. Other constructions have been proposed, in micromachined silicon, and in alloys based on gallium arsenide.
These filters are generally produced on a wafer. More precisely, the filters are produced collectively on a transparent substrate, such as for example indium phosphide. The notion of transparency applies, of course, to the wavelength of the band in question. It is possible to produce several hundred filters on one and the same substrate wafer.
Filters formed by two Bragg mirrors separated by an airgap are extremely fragile. Their thickness does not exceed a few microns and to handle them is consequently very tricky.
The object of the invention is to alleviate this problem by proposing a process for the collective fabrication of optical filtering components which allows easier handling.